Feat v0.6.0 cluster registry (#36)

Co-authored-by: Jeffrey Smith <jasafpro@gmail.com>
Co-committed-by: Jeffrey Smith <jasafpro@gmail.com>

server/database/migrations/postgres/013_cluster_registry.sql

@@ -0,0 +1,13 @@
+-- 013_cluster_registry.sql — v0.6.0
+-- PG-backed cluster registry for node self-assembly.
+-- UNLOGGED: no WAL overhead, contents lost on PG crash (correct behavior —
+-- all nodes are dead anyway and re-register on restart).
+
+CREATE UNLOGGED TABLE IF NOT EXISTS node_registry (
+    node_id       TEXT PRIMARY KEY,
+    endpoint      TEXT NOT NULL DEFAULT '',
+    seq           SERIAL,
+    registered_at TIMESTAMPTZ DEFAULT now(),
+    heartbeat     TIMESTAMPTZ DEFAULT now(),
+    stats         JSONB DEFAULT '{}'
+);